CUI (Corrosion Under Insulation) has always been a challenge for plant operators,quality assurance/reliability engineers and equipment owners. It is hard to identify the problem until it has become an emergency situation, often leading to unit shutdowns or even the whole facility shut down for emergency repairs.As per industry statistics, next to leakages in flanged joints, the highest incidents of piping failure in process industry is caused due to corrosion in pipe, especially under insulation.An insulated piece of equipment can have trapped moisture by two means: moisture can become trapped due to cyclic equipment and condensation forming and being trapped under insulation. The second and more common reason is rain/ snow.Rainwater or water from melting snow will eventually enter weak points in the system and pour onto the surface under insulation. The difference when compared to a stack is that the water gets trapped because of the insulation and is not allowed to escape.Thermal shock can also be a significant cause of CUI. Thermal shock is categorized by a dramatic rise or fall in the temperature of the equipment. Thermal shock may occur when a unit is turned on or off, during the normal cyclic conditions the unit may exhibit, or when the steel is exposed to water that has penetrated the jacketing.